1. Field of the Invention
The present invention relates to an image forming apparatus that has a light emitting element for emitting light and a light receiving element for receiving reflected light of the light emitted from the light emitting element as means for detecting presence or absence of a sheet on a sheet conveying path.
2. Related Background Art
In a conventional image forming apparatus, a photo-interrupter, which is a mechanical detecting method, is used as means for detecting a sheet on a sheet conveying path. The photo-interrupter has a movable plate between a light emitting element and a light receiving element in a photocoupler, and light from the light emitting element side can reach the light receiving element side without being blocked when the photo-interrupter does not detect a recording sheet. Thus, a constant voltage is outputted.
In contrast to this, when the photo-interrupter detects a recording sheet, the recording sheet bumps against the movable plate to move the plate, and light emitted from the light emitting element side is blocked before the light reaches the light receiving element side. Thus, no voltage is outputted. Therefore, the image forming apparatus can judge presence or absence of the recording sheet according to presence or absence of an output voltage in both the cases. Then, immediately after the recording sheet passes the photo-interrupter, the movable plate is apt to return to its original position by a force of a spring or the like. At this point, the movable plate returns to the original position while being vibrated by a reaction of the spring, and an output level of the light receiving element fluctuates around a level where the photo-interrupter can detect presence or absence of the recording sheet. As a result, the image forming apparatus cannot accurately detect whether the recording sheet is present or not.
In order to feed sheets at high speed and with accuracy, it is necessary to keep an interval between the sheets constant. For this purpose, the image forming apparatus has to judge a leading end and a trailing end of a recording sheet accurately. However, if chattering due to mechanical vibration as in the above-described photo-interrupter occurs, the trailing end of the recording sheet cannot be detected accurately.
In order to prevent such a disadvantage and detect presence or absence of a sheet at high speed, a reflective optical sensor has been used in an image forming apparatus. The reflective optical sensor utilizes a phenomenon that an output differs depending on a difference in a reflection intensity between a recording sheet and a plate of a material with high reflectivity. Thus, this sensor does not come into contact with the recording sheet and makes it possible to detect the recording sheet at high speed.
However, the reflective optical sensor has a disadvantage that an output value fluctuates. As to a factor of the fluctuation of the output value, it can be considered that a light emitting element and a light receiving element deteriorate due to wear and reflectivity of a reflection plate deteriorates by sheet powder when a sheet is fed.
In optical sheet detecting means, the more an electric current is flown, the higher luminance a light emitting element has and the larger dynamic range with respect to presence or absence of a medium can be secured. Thus, reliability of detection accuracy is improved. However, since an electric current value is increased, the service life of the light emitting element is reduced. In the case of the photo-interrupter, although it may also be referred to as optical in that a photo-coupler is used, it is provided with light emitting and light receiving elements within a short distance, and moreover, can surely detect even a very small amount of light when shielded by a black material that is not susceptible to reflection. In contrast to this, in an optical sheet sensor, since a shielding medium itself may have high transmissivity or high reflectivity, it is required to secure an amount of light that can be distinguished surely. Therefore, if the amount of light is set low, this is advantageous for the service life of the light emitting element but the dynamic range is narrowed. Thus, it is likely that the medium and stain affect the sensor more adversely and the sensor performs wrong detection. As a measure for coping with this problem, conventional means is used which adjusts an amount of emitted light of a reflective optical sensor when a main power source of an image forming apparatus is inputted to make an output voltage constant.
For example, when the main power source is inputted, if adjustment of an amount of light of the light emitting element is performed in a state in which a sheet is present at a detection position of the reflective optical sheet sensor, light received by the light receiving element becomes reflected light from the sheet. Since an amount of this reflected light is small, the amount of light of the light emitting element is increased more than necessary in order to keep the amount of the reflected light at a predetermined value. As a result, an electric current flowing to the light emitting element becomes excessive, which is a cause of decreasing the service life of the element.
In addition, in a method of adjusting an amount of light only at the time of input of the main power source for coping with deterioration of characteristics during the operation of the reflective optical sheet sensor, an interval of the adjustment is too long and the adjustment may be insufficient in a high-speed machine that prints a large number of sheets. That is, because a large amount of printing is executed particularly in a high-speed image forming apparatus since the main power source is inputted until the power source is cut off and the next power source is inputted, sheet powder generated during the conveyance of sheets accumulates on the light emitting element and the light receiving element of the reflective optical sheet sensor. As a result, accuracy of detecting sheets falls, which may become a cause of wrong detection of jam.
Moreover, assuming that a system for adjusting an amount of light before and after a job is employed, if the number of sheets to be printed in one job is too many, sheet powder generated during the conveyance of the sheets accumulates on the light emitting element and the light receiving element of the reflective optical sheet sensor in the same manner as described above. As a result, accuracy of detecting sheets falls, which may become a cause of wrong detection of jam.
Further, since adjustment of all optical sheet sensors is always performed, the adjustment takes a relatively long time in an apparatus having a plurality of optical sheet sensors.
According to an embodiment of the present invention, there is provided an image forming apparatus including:
a sheet containing section for containing sheets;
a conveying member and a conveying path for conveying the sheets;
at least one optical sensor that is arranged in the conveying path and has a light emitting element and a light receiving element for detecting presence or absence of a sheet on the conveying path; and
a driver for changing an amount of emitted light of the optical sensor, in which
the apparatus includes:
a sheet supply detecting means for detecting a supply operation of the sheets contained in the sheet containing section; and
a control section for adjusting an amount of emitted light of the optical sensor according to an output of the sheet supply detecting means.
According to another embodiment of the present invention, there is provided an image forming apparatus including:
a sheet containing section for containing sheets;
a conveying member and a conveying path for conveying the sheets;
at least one optical sensor that is arranged in the conveying path and has a light emitting element and a light receiving element for detecting presence or absence of a sheet on the conveying path; and
a driver for changing an amount of emitted light of the optical sensor, in which
the apparatus includes:
a counter for counting sheets every time a sheet passes through the optical sensor; and
a control section for adjusting an amount of emitted light of the optical sensor according to judgment on whether a value of the counter reaches a predetermined value.
According to still another embodiment of the present invention, there is provided an image forming apparatus including:
a sheet containing section for containing sheets;
a conveying member and a conveying path for conveying the sheets;
at least one optical sensor that is arranged in the conveying path and has a light emitting element and a light receiving element for detecting presence or absence of a sheet on the conveying path;
a driver for changing an amount of emitted light of the optical sensor; and
a control section for judging whether or not an amount of emitted light of the optical sensor is adjusted, and for, even with the judgement that the amount of emitted light of the optical sensor is to be adjusted, stopping the adjustment of the amount of emitted light of the optical sensor in the case where the optical sensor detects presence of a sheet on the sheet conveying path.
The other objects and features of the present invention will be apparent from the following description and drawings.